Also consider using a shoulder bolt, a bolt that is not threaded all the way. If your shearing the bolts off this will increase the amount of metal in the area they are shearing. In a perfect world a bolt should be threaded as much as is required since the threaded area is the weakest point. I see way to many people using tap bolts (threaded the entire length) when they should be using shoulder bolts.
Trust me on the grade 9 frame bolts or flange bolts, they are designed to take extreme shear stress.
CCWKen - You state "The grade rating is a measure of material strength not hardness". Actually the "grade" is nothing more than a name or nomenclature for a specific set of specification's like a SAE, ASTM or MIL spec.
SAE J429-Grade 5 are medium carbon steel, quenched and tempered. Min. Proof Strength 85,000 psi up to 1" diameter. Min. Tensile Strength 120,000 psi up to 1" diameter.
SAE J429-Grade 8 are medium carbon alloy steel, quenched and tempered. Min. Proof Strength 120,000 psi. Min. Tensile Strength 150,000 psi.
FNL Grade 9 are medium carbon alloy. Min. Proof Strength 140,650 psi. Min. Tensile Strength 180,000 psi.
The Industrial Fastener Institute (Inch Fastener Standards, 7th ed. 2003. B-2) states that shear strength is approximately 60% of the minimum tensile strength.
"As an empirical guide, shear strengths of carbon steel fasteners may be assumed to be approximately 60 percent of their specified minimum tensile strengths. For example, an SAE grade 5 hex cap screw has a specified minimum tensile strength of 120 ksi. Therefore, for design purposes, its shear strength could be reasonably assumed to be 72 ksi."
Let's look at an example of where Gr.5, Gr.8 and Gr.9 bolts are subjected to single shear loads.
A = Cross-sectional area of the fastener size (since bolt bodies/shanks have circular cross-sections, use area of a circle) = Pi x r2 where R (radius) = .250/2 = .125, therefore A = Pi x (.125)2 = .0491 square inches (in2)
Using a .250-inch diameter grade 5 fastener gives you the following shear capability:
Capability in shear = 72,000 lbs / in2 x .0491 in2 = 3535 lbs
Using a .250-inch diameter grade 8 fastener gives you the following shear capability:
Capability in shear = 90,000 lbs / in2 x .0491 in2 = 4419 lbs
Using a .250-inch diameter grade 9 fastener gives you the following shear capability:
Capability in shear = 108,000 lbs / in2 x .0491 in2 = 5,302 lbs
That's a difference of over 1,700 lbs or almost 1 ton. In this example you can clearly see that using a grade 8 or grade 9 fastener has a superior advantage over the grade 5. Therefore the result is if someone is using grade 5 bolts in a shear application they will fail 884 lbs earlier than a grade 8 and fail 1,767 lbs earlier than a grade 9.
I've also heard the argument that grade 8's are more brittle than grade 5's and that's why you shouldn't use them. First you need to understand what the term "brittle" really means. Brittleness in bolts is defined as failure at stresses apparently below the strength of the bolt material with little or no evidence of plastic deformation. Typically, fasteners are not brittle below 180 ksi ultimate tensile strength. Grade 5's have an ultimate tensile strength of 120 ksi, a grade 8 fastener has an ultimate tensile strength of 150 ksi and a grade 9 fastener has a ultimate tensile strength of 180 ksi. This is why brittle is a relative term. Nearly all fasteners are considered ductile except some made from PH 15-6 Mo, 17-4 PH and 17-7 PH.
Toughness is an important feature of a fastener. It is the opposite of brittleness and gives you an idea of how it will handle abuse without being damaged and eventually weakening the fastener or can cause fatigue to appear much earlier than normal. One way to "measure" toughness is by looking at the hardness rating of a fastener. The higher the number (Brinell, Rockwell) the harder the material is and the tougher it is to damage. According to Marks Standard Handbook for Mechanical Engineers, Grade 5 typically have a core Rockwell hardness of C25-C34, a grade 8 typically has a core Rockwell hardness of C33-C39 and a grade 9 typically has a core Rockwell hardness of C33-C39 (same as a grade 8). Based on this, grade 8's and 9's are tougher than grade 5's.
Fatigue usually doesn't play a big part in grade 9, 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles. Here is a quick point about fastener fatigue. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure. Therefore, you want to make sure you torque your fasteners to the appropriate level using a torque wrench and make sure to torque dry, clean threads. Lubricated threads significantly change the actual preload on the fastener and you risk over torquing it.
